(1) Field of the Invention
The present invention relates to a manufacturing method of an image sensor comprising an imaging area which includes a plurality of pixel units and a wiring area around the imaging area, and in particular, to the improvement of a shading characteristic.
(2) Description of the Related Art
Recently, the sensitivity of an image sensor has been improved by forming a micro lens on each pixel and collecting an incident light on each of a plurality of pixel units.
FIG. 1 is a top view of such an image sensor as described above. In FIG. 1, the image sensor comprises an imaging area 2 including a plurality of pixel units (a photodiode), a wiring area 3 around the imaging area 2, and an antireflective film 4. The antireflective film 4 is a lightproof filter of low transmittance (e.g. black), formed on the wiring area 3 surrounding the imaging area 2, and is equipped so as to prevent flares caused by a reflected incident light. Here, flares refer to light reaching a photodiode that causes a low-contrast image of a subject or a turbidity of colors, the light for contributing to imaging the subject, such as a reflected light and a scattered light occurring on the surface of the photodiode or in a path of an incident light.
Japanese Laid-Open Patent application No. H9-293848 discloses a technology in which the occurrence of a flare is decreased by forming a scattered reflective film on the antireflective film 4 and decreasing the reflectivity around the imaging area 2.
However, the problem with the conventional technology is that a step height is generated between the imaging area 2 and the area above the wiring area 3 including the antireflective film 4 for avoiding a flare, especially around the imaging area, thus causing an uneven sensitivity (shading), and thereby degrading image quality.
This problem will be explained in further detail using FIG. 2. FIG. 2 is a section view of the ambient imaging area of an image sensor of the conventional technology. As FIG. 2 shows, there is a wiring area 3 around the imaging area 2 including a plurality of pixel units (photodiode 5). The manufacturing processes are as follows: i) the wiring area 3 is flattened, ii) an anti-flare pattern of low transmittance (an antireflective film, a lightproof film) is formed on the wiring area 3, iii) a transparent film 8 such as acryl is applied as necessary, and iv) micro lenses are formed on the transparent film 8.
According to this structure, a step height is generated in the ambient part (8a in FIG. 2) which exists from the imaging area 2 to the wiring area 3. In other words, the distance between the photodiode 5 and the micro lenses is different at the central of the imaging area 2 than at the ending part of the imaging area 2. In the ending part of the imaging area 2, the distance between the micro lenses and the photodiode 5 deviates from the optimum design value, and therefore the photodiode does not acquire enough incident light. Consequently, an uneven sensitivity (shading) is created, thereby degrading image quality.